1. Field of the Invention
The invention relates to a method and apparatus for the transfer of rotary torque drilling energy from a power source to a drilling tip. More particularly the invention relates to method and apparatus of the character described for drilling a tight radius curved bore hole with a flexible rotary drill shaft.
2. Description of the Prior Art
Many rotary drive apparatus exist for different particular purposes. Each flexible shaft rotary drive is designed to operate at acceptable levels for specific purposes and to accomplish operating parameters of certain specificity. None of the prior art referenced herein have been made to operate at requirements set by drilling a curved bore in a hard material. For example, most prior art reviewed and cited by applicant, although related to flexible rotary drill shafts and wire rope members, are cited for specific purposes unrelated to boring curved holes. These devices operate at very high RPMs and generally have different operating performance requirements. One such example is U.S. Pat. No. 4,686,982 to Nash which discloses a spiral wire bearing for rotating a wire drive catheter. This disclosure represents a typical type of flexible drive means which has no center core and is made to operate at very high speed, generally in excess of 20,000 RPMs. This basic design combination would fail in the drilling apparatus of the present invention due to the extreme loads encountered by the center of the present drilling energy transfer member (DETM).
Another type of prior art is a typical industrial flexible shaft configuration such as disclosed in U.S. Pat. No. 5,820,464 to Parlato which has a mandrel and six total layer wraps. These particular types of flexible shafts are made with multiple wraps of very small wires with the center comprising multiple wraps laid at a very steep helix pitch. The problem with this type of flexible shaft is that it is generally wrapped too tight and it is too stiff for the application of the present invention. Having been wrapped too tight and with a too stiff helical pitch at the center, it does not possess sufficient longitudinal strength or flexibility along the center to withstand high torque loads when passing around sharply curved radiuses.
Another such prior art example is disclosed in U.S. Pat. No. 4,185,473 to Troost. Troost discloses another example of too many lays or wraps, too many wires wrapped at too steep of a pitch angle. Also, another problem with the Troost shaft is that the pitch is too steep in the center wraps which are not laid for longitudinal linear strength along the center. This flexible shafting would also fail when put in the present operating environment since the strands are wrapped too tightly and the lack of flexibility would not allow for transmitting drilling energy around a tight curve radius. These types of flexible shafts generally have multiple wraps of wires that are substantially the same diameter and therefore are not balanced properly to handle the extreme loads experienced in multiple operating positions such as tight curves and straight operating runs.
The U.S. Pat. No. 5,052,404 to Hodgson discloses another type of torque transmitting device. This particular torque transmitter simply has too few coils and is therefor not flexible enough to withstand drilling around a tight radius. Another type of rotary transmitter is a rotary transmission conduit such as disclosed in U.S. Pat. No. 5,072,759 to Moore. The Moore transmission conduit includes an inner tubular liner comprised of polymer material and an outer layer of adhesive material. These types of devices usually comprise conduit made up of sub-assemblies of different types of wire wraps with additional component configurations. The problem with this type of devices is that the sub-assembly componentry takes up valuable space which reduces strength to below that required in a highly flexible curved drilling application. Yet another type of device is described in U.S. Pat. No. 5,165,421 to Fleischhacker et al. Fleischhaker et al discloses a lumen cable which is formed from helically wound inner and outer coils. The problem with this type of configuration is that no tensile or linear compression components exist within the structure and, as a result, failure will occur rapidly if linear stress is applied. All of these basic prior art designs would fail in the present tight radius curved bore drilling device because of the lack of balance, flexibility requirements and the extreme loads placed on the center of the DETM. Also there are balancing forces that are required in the DETM which must perform under multiple different positions during loading.
Wire rope is another type of prior art stranded configuration of some relevance. These configurations are manufactured primarily for linear travel over pulleys and are intentionally constructed so as not to rotate as they ride over a pulley. This approach teaches in the opposite direction from the present invention.
Finally my prior U.S. Pat. Nos. 5,700,265; 5,509,918; 5,002,546; and 4,941,466 represent prior art flexible shafting that has been used in a tight radius curved drilling. The problem with these rotary drives is that they do not balance the outer extensor and the inner compressor forces and hence have a reduced operating life. Their center configuration is not laid to withstand the necessary tensile loads and the outer torque layers are not wrapped for balancing the two outer lays with respect to each other and with respect to the center tensile compressor lays. Therefore, the operating life of this type of rotary drive is reduced.
Transmitting drilling energy along a drilling energy transfer member (DETM) between an energy source and a working tip for the purposes of drilling a tight radius curved bore presents unique operating requirements. Not only does the DETM have to operate in multiple operating positions, i.e. between curved and straight runs, it must carefully balance the net reaction forces that occur between the energy source and the multiple and variable opposing reaction forces encountered in drilling a tight radius curved bore. In general the overall work zone of the DETM includes: (1) rigid attachment at one end to a working tip; (2) travel along a short straight section; (3) travel along a tight radius curve; (4) transition from the tight radius curve to a straighter section; (5) translation up into a straighter self supporting section; and (6) then attachment to a solid power shaft. The net reaction forces of the DETM must be carefully balanced to successfully operate in these specific dynamic work zones. This includes balancing: (1) the vector forces at the cutter attachment; (2) vector forces at the transition between the cutter attachment and the curve; (3) the vector forces through the curve; (4) the vector forces at the peak stress area within the curve; (5) the vector forces at different amounts of curved radius and changing of the peaked stresses; (6) the vector forces coming off the peaked stresses and transitioning into the straight section; (7) the vector forces in the self supporting straight section; and (8) the vector forces where the DETM terminates at its attachment with a proximal solid shaft. Balance among all of the vector force relationships in the context of load sharing is also very important.
There are a number of important characteristics that must be considered when manufacturing a DETM that will operate in the aforementioned environment. Some of these important characteristics include: the number of wires; degree of cold work temper; the number of wires per wrap; the optimum stranded pitch; the optimum operational pitch; the pitch excursion off center of mass of the wires as the DETM rotates; stress relieving the wires by heat tempering after stranding; selection of the correct wire size; selection of the correct wire size percentage relative to the overall diameter of the wire and the wrap space; percentage of space within the wrap; the percentage of the diameter relative to the radius; the transition zone; the vector force patterns in a straight near the crimp; the vector force patterns in the curve at the peaked radial position; the vector force pattern excursion flexibility during transition; the vector force pattern at the laser weld straight at the end of the curve; the strand excursion side-to-side; the radial excursion; the wrap excursion between layers; the difference of excursion at the three o""clock, six o""clock, nine o""clock and twelve o""clock positions of rotation. Other considerations that must be made relate to: the peak stress areas; the laser weld termination of flexibility area; the heat affected zone control area of the laser weld; the peak in the curve; the translation of the peak drilling stresses as a DETM translates into a greater portion of a curved position and then back to the peak stresses in a straight unsupported position; and the peak forces at the proximal rigid terminal end at the crimp.
The present invention provides a flexible drilling shaft and method of constructing the same which balances the net action/reaction forces that occur between the drilling energy source and a working tip, especially when drilling in a tight radius in extremely hard or dense material. Balance is maintained as the shaft operates between and through curved and straight runs in forming a tight radius bore. A center or core load cell provides tensile and compressive strength and comprises a plurality of strands that are sized and laid at helical angles sufficient for transmitting predetermined axial loads under rotary drilling pressure. An outer wrap load cell provides torque and rotational strength and comprises a plurality of strands that are sized and laid over the core at helical angels sufficient for transmitting predetermined torque loads under rotary drilling pressure. The force fields and mass distribution of the core and outer wrap load cells are functionally balanced such that the core load cell structurally supports the outer wrap load cell against destructive axially directed forces and the outer wrap load cell structurally supports the core against destructive rotationally directed torque forces and maintains longitudinal support therefor.
In a preferred embodiment having a 0.045 inch flexible drilling shaft designed for drilling xc2xc inch curved radius bore holes in such hard material as bone, a shaft configuration of 1xc3x9719+5+7 is provided. The flex shaft is constructed by first laying six wire strands in a right hand direction about a single wire mandrel and then laying a twelve strand wire wrap at the same helical angle in the opposite direction to form a first or core load cell. These strands are laid generally axially at a relatively flat helical angle of from 10xc2x0-15xc2x0 for the purpose of transmitting tension and compression loads during rotary drilling. A second load cell is formed about the core and comprises a five strand right hand wrap and a seven strand left hand wrap laid at 60xc2x0-68xc2x0 and 68xc2x0-72xc2x0 respectively and serves to transmit torque loads during rotary drilling. The core load cell and the outer wrap load cell are functionally balanced with respect to mass and the forces contained within the flexible shaft, providing superior axial strength with the torque carrying wraps maintaining overall structural integrity of the shaft during tight radius curved bore forming.
An improved attaching means for rigidly connecting the distal end of the flex shaft to a cutter head is provided comprising, in a first embodiment, a hollow stem on the cutter head having a diameter adapted to receive the shaft end. The shaft end is then laser welded or otherwise fusibly connected to the stem to provide the rigid connection. In a second embodiment the cutter head stem is equal in diameter with the shaft end and a separate sleeve is provided to span the abutted stem and shaft ends. In this embodiment welding may be accomplished on one end of the sleeve adjacent the cutter head, providing an undisturbed bearing surface for contact with a drill guide.